1. Field of the Invention
The present invention relates to a filter circuit using a transistor, and a filter integrated circuit formed on an integrated circuit.
2. Description of the Related Art
In an optical communication system or a radio communication system using high speed signals of several gigabits per seconds [Gb/s] or higher, high-pass, low-pass, and band-pass filters which operate in a high frequency band of several gigahertz [GHz] or higher are sometimes required. For example, in a timing circuit limiter amplifier for a 3R type optical repeater, a band-pass amplifier or the like using a tuning circuit and so forth is employed so as to suppress jitter caused by circuit noise and to prevent an output waveform from being linked by harmonics.
Generally, a filter circuit requires inductance as well as resistors and capacitors. However, it is difficult to form inductance in an integrated circuit (IC) device. Thus, a filter circuit is constructed by connecting an external coil. Alternatively, the entire filter circuit is used as an external part.
However, when inductance is provided as an external part, as the-frequency increases, stray capacitance, stray inductance, and so forth increase. As a result, the characteristic of the filter in a high frequency band thereof degrades.
To solve this problem, a filter is constructed of an operational (OP) amplifier, capacitors, and resistors which can be used for an IC device. This type of filter is referred to as an active filter.
FIGS. 1A and 1B are schematic diagrams showing bridge T type circuits. FIG. 2 is a schematic diagram showing a circuit of a band-pass filter which is constructed of a bridge T type circuit and an OP amplifier.
FIG. 3 is a schematic diagram showing a circuit of an active high-pass filter using an OP amplifier. FIG. 4 is a schematic diagram showing a circuit of an active low-pass filter referred to as a Sallen-Key circuit.
In the above-mentioned active filter using an OP amplifier, since the characteristic of the filter depends on the frequency characteristic of the OP amplifier, the filter can operate at up to several megahertz [MHz] rather than in a high frequency band of several gigahertz [GHz] or higher.
To solve this problem, a conventional wide-band amplifier instead of the OP amplifier can be used. However, even with a wide-band amplifier, since the phase characteristic in the high frequency band varies, a desired filter characteristic may not be obtained or an oscillation lay take place.
As shown in FIGS. 3 and 4, an active filter using a voltage follower which is constructed of an emitter follower circuit of a transistor can provide a wide-band filter although it cannot be applied to the band-pass filter using a negative feedback circuit of an OP amplifier as shown in FIG. 2.
FIGS. 5 and 6 are schematic diagrams showing circuits of a high-pass filter and a low-pass filter, each of which uses an emitter follower. In these filters, the OP amplifiers shown in FIGS. 3 and 4 are substituted with emitter follower circuits which are constructed of transistors.
However, in the Sallen-Key low-pass filter using the emitter follower shown in FIG. 6, when the filter operates in a high frequency band of gigahertz [GHz] or higher, a designed filter characteristic cannot be obtained due to the influence of the capacitance and resistance of the transistor.
Next, the influence of resistance, capacitance, and so forth of a transistor over a Sallen-Key low-pass filter using an emitter follower circuit shown in FIG. 6 will be described.
FIG. 7 is a schematic diagram showing an equivalent circuit, when operating in a high frequency band, of the Sallen-Key low-pass filter using the emitter follower circuit shown in FIG. 6. As shown in the FIG. 7, a base resistance R.sub.b, an emitter resistance R.sub.e, a base-collector capacitance C.sub.bc, and so forth of the transistor are added to the low-pass filter shown in FIG. 6. Thus, in the circuit shown in FIG. 7, the low-pass filter which consists of R.sub.1, R.sub.2, C.sub.1, and C.sub.2 is connected to the low-pass filter which consists of R.sub.b and C.sub.bc in series. As a result, when the frequency band of the filter which consists of R.sub.1, R.sub.2, C.sub.1, and C.sub.2 is much lower than that of the filter which consists of R.sub.b and C.sub.bc, a desired frequency characteristic can be obtained. However, when a low-pass filter which operates in a much higher frequency band is required, the frequency characteristic of the filter which consists of R.sub.b and C.sub.bc cannot be ignored.
Moreover, in a high frequency band, since the emitter resistance R.sub.e influences the output impedance of the emitter follower circuit, the low impedance intrinsic to the emitter follower circuit cannot be obtained.
FIG. 8A and 8B are schematic diagrams and graphs showing equivalent circuits and gains in a high frequency band (in the vicinity of the cutoff frequency) of a Sallen-Key low-pass filter. FIG. 8A shows an equivalent circuit and a gain in the ideal case where the filter is not affected by the parameters of the transistor. FIG. 8B shows an equivalent circuit and a gain in the case where the filter is affected by the emitter resistance R.sub.e of the transistor. In the equivalent circuits of FIG. 8, in the high frequency band, it is assumed that the capacitor of the filter is shorted.
At the cutoff frequency or high of the filter, the ideal low-pass filter shown in FIG. 8A has a particular attenuating characteristic. On the other hand, in the case where the influence of the emitter resistor R.sub.e of the transistor cannot be ignored as shown in FIG. 8B, at the cutoff frequency or higher of the filter, the gain of the filter is larger than a particular value. Thus, in this case, the filter does not have the characteristic of a low-pass filter.
FIG. 9 is a graph showing the frequency characteristic of the conventional low-pass filter using the emitter follower of FIG. 6. The solid line of the graph represents calculated values of an ideal characteristic. The dot line of the graph represents a simulated result using real transistor parameters.
As shown in FIG. 9, at around six gigahertz [6 GHz] before which is lower than the cutoff frequency, the output of the filter temporarily drops due to the influence of the base-collector capacitance C.sub.bc and the base resistance R.sub.b of the transistor. Thereafter, the stopping characteristic of the filter in the cutoff region degrades due to the influence of the emitter resistance R.sub.e of the transistor. As a result, a low-pass filter using an emitter follower cannot be used in a high frequency band of several gigahertz [GHz] or higher.
In addition, a circuit has been necessarily integrated in considerations of cost and reliability. In particular, the shorter the wavelength of a signal in a high frequency band, the more the analysis of a distributed constant of the circuit becomes important. Thus, the filter should be formed on an IC device.
When a filter is formed on an IC device, the frequency characteristic of the filter should be variably controlled so as to change the frequency band thereof and to compensate for the characteristic deviation which takes place in the production stage from device to device.
As a related art which can solve the above-mentioned problem, for example Japanese Patent Laid-Open Ser. No. 1984-215111 is known, which discloses a technology of adjusting a filter characteristic by changing a resistance of a diode after replacing a resistor used in a Sallen-Key type filter by the diode so as to adjust the operating characteristic of the filter.
However, it is likely that the object of the above-mentioned related art is to adjust the characteristic of the Sallen-Key low-pass filter rather than adjust the frequency characteristic of a filter which can operate in a high frequency band of several gigahertz [GHz] or higher.